1. Field of the Invention
The present invention relates vehicle engine starting systems.
2. Description of the Problem
Designers of internal combustion piston engines have appreciated since their inception that such engines can function as pumps. Engine compression brakes exploit this aspect of engines to advantage. A compression brake operates by opening the cylinders"" exhaust valves at or just before top dead center (TDC) of the pistons"" cycles. Thus the energy used to compress the air in the cylinders is lost through the exhaust valves and no energy is returned to the crankshaft through the pistons during the expansion portions of the piston strokes. On motor vehicles this allows a substantial portion of an engine""s rated power to be applied to braking.
Compression brake systems are implemented by installing controls for exhaust valves on many diesel engines. In one type of engine compression brake, the Jacobs engine brake, the exhaust valves are normally actuated with a standard camshaft. Normal actuation can be interrupted during braking by using energy from the injector push rod to open the exhaust valve for a cylinder at TDC of the piston. A detailed explanation of the principal of engine compression brakes, an in particular Jacobs engine brakes, may be found at pages 736-744 of Electric and Electronic Systems for Automobiles and Trucks by Robert N. Brady, Reston Publishing Company, Inc., Reston, Va. (1983).
Exhaust valve control has also been used on kick start motorcycles. Compression rebound occurring during attempts to start motorcycle engines could be dangerous to the riders. Compression rebound was caused when energy stored compressing an air mass in a cylinder was returned to the engine crankshaft during a down stroke of the piston. While less energy is returned than was put into the system compressing the air, the force can still be substantial. For that reason, prior to the widespread use automatic starters on motorcycles, some motorcycles came equipped with a manually activated valve switch, which allowed operators to roll the engine to top dead center (TDC) before attempts were made to kick start the vehicles"" engines and thereby avoid compression rebound.
Another area where the pump aspects of engines has undesirable results is in starting diesel engines. Diesels, which typically do not have ignition sources inside the cylinders, rely on compression heating to bring the fuel air mixture to its flash point. During cold weather, the engine block can serve as a substantial heat sink, meaning that compression must be reasonably fast to assure that the mixture reaches the ignition temperature before the temperature drops due to heat loss to the engine and to the environment. Typically, diesels must be cranked to 100 rpm before combustion can occur. Diesels have substantially higher compression ratios than do gasoline engines and require more energy input to compress the air in the cylinders than in gasoline engines. All of these aspects of diesels make engine cranking high on transient energy consumption. Starter motor power consumption can reach 20,000 watts at xe2x88x9220 degrees Fahrenheit with engine manufacturers"" recommended oil weights. Such power demands impose high loads on starter systems, starter motors and batteries, necessitating the use of large battery plants and large, heavy duty starter motors. Incomplete and failed combustion during cranking contributes to high levels of emissions releases during start up, particularly when the engine is cold.
The assignee of the present invention has implemented compression brakes on its diesels in a different manner than in the classic Jacobs engine brake. In the assignee""s diesels, a hydraulic pump supplies engine oil at high pressure to the injectors and to exhaust valve override actuators as soon as the engine begins turning over. The exhaust valve override actuators are electronically controllable, allowing the exhaust valves to be opened at any point in the piston stroke for four stroke diesel engines.
One object of the invention is to reduce power consumption during cranking and starting of internal combustion engines.
Another object of the invention is to reduce emissions during engine starts.
Still another object of the invention is to reduce power demands on starter motors and starter motor circuitry, allowing commensurate reductions in motor size and output.
Yet another object of the invention is to reduce the size of the battery plant.
One aspect of the invention is to provide a method of boot strap starting an internal combustion engine having a plurality of cylinders. Upon initial cranking of the engine the exhaust valves for each cylinder are opened during piston compression strokes. Upon engine rotational speed reaching a first minimum threshold, the exhaust valve for one cylinder is allowed to open and close normally in synchronous with movement of a piston in the cylinder. As combustion commences in the one cylinder operating normally, the remaining exhaust valves are allowed to open and close in synchronous with movements of pistons in their respective cylinders. Also responsive to combustion in the first cylinder, cranking of the engine by the starter motor is discontinued.
Additional effects, features and advantages will be apparent in the written description that follows.